The present invention relates to screening apparatus for screening of articles and, more particularly, relates to a screening apparatus for screening ceramic greensheets in which the ceramic greensheets may be screened multiple times without losing registration with the screening apparatus.
In the current manufacture of multilayer ceramic (MLC) substrates for integrated circuit semiconductor package structures, a plurality of ceramic sheets is formed by doctor blading a slurry containing a resin binder, a particulate ceramic material, solvents, and a plasticizer, drying the doctor bladed sheet, and blanking it into appropriately sized greensheets. Via holes are then mechanically punched for forming electrical interconnections through the greensheet. Electrically conductive paste is screened in the via holes, and in appropriate patterns on the surface of the greensheets, the greensheets stacked and aminated, and the assembly subsequently fired at an appropriate sintering temperature.
Critical to the manufacturing process is the screening operation since the ceramic greensheets are relatively fragile, their thickness may be on the order of 0.008 inches, and their surface area is relatively large compared to their thickness. As will be appreciated by those skilled in the art, particular problems are encountered when screening such relatively fragile, paper thin, flexible greensheets which are of no concern to the general screen printing art utilizing rigid substrates.
A screening mask is precisely aligned with the greensheet so that when the electrically conductive paste is screened in the via holes and in appropriate patterns on the surface of the greensheets, the screened via holes and patterns are where they are supposed to be.
The screening operations of filling via holes and screening patterns are often done simultaneously, thus the screening paste has to be tailored to satisfy both the via fill and pattern deposition requirements. This approach is not without its problems since the screening paste is a compromise between the differing requirements of via fill and pattern deposition.
An alternative approach is to fill the via holes with a paste Theologically tailored for vias, dismount the greensheet from the screening stage, dry it, remount it on the screening stage, align the greensheet with the screening mask for the patterns, and then screen paste that has been tailored for patterns. This approach presents problems with sheet distortion that occur during the drying process, making greensheet realignment difficult or sometimes impossible for the second screening operation.
Various screening apparatus have been proposed by those skilled in the art. One particular screening apparatus is that disclosed in Davis et al. U.S. Pat. No. 4,362,486, the disclosure of which is incorporated by reference herein. This reference discloses a screening apparatus in which there are two separate loading and screening stations but only one unloading station. Two ceramic greensheets may be loaded and screened at the same time but they must be unloaded alternately as there is only one unload station. To move from one screening station to the other, it is necessary to dismount the greensheet to the unloading station, dry the greensheet, and then remount it at the second screening station.
It would be desirable to be able to move from one screening station without the necessity of dismounting the greensheet and then remounting it.
Accordingly, it is an object of the present invention to have a screening method in which a ceramic greensheet may be moved from one screening station to another without dismounting the ceramic greensheet from the screening stage.
It is another object of the present invention to have a screening method which allows the screening of a paste tailored for the filling of via holes and the screening of a second paste tailored for the deposition of patterns.
It is yet another object of the present invention to have a screening method wherein a ceramic greensheet is displaced from a first position to a second position while remaining in place on a screening stage so that realignment of the ceramic greensheet to the screening stage is unnecessary.
These and other objects of the present invention will become more apparent after referring to the following description of the invention in conjunction with the accompanying drawings.
The purposes of the invention have been achieved by providing, according to a first aspect of the invention, a method for displacing an article using first and second pallets for holding and transporting an article between two process stations and an H-bar assembly for receiving the first and second pallets, the method comprising the steps of:
receiving the first and second pallets and articles thereon by the H-bar assembly at one of the two process stations;
performing a first process on the articles while received by the H-bar assembly;
moving the first and second pallets and articles thereon to a second process station; and
performing a second process on the articles while at the second process station.
According to a second aspect of the invention, there is provided a method for displacing an article using first and second pallets for holding and transporting an article from a first position to a second position and a rotatable H-bar assembly for receiving the first and second pallets at the second position, the method comprising the steps of:
moving the first and second pallets and articles thereon from the first position to the second position;
receiving the first and second pallets and articles thereon by the H-bar assembly at the second position;
rotating the H-bar assembly a predetermined amount so as to exchange positions of the first and second pallets and articles thereon; and
moving the first and second pallets and articles thereon from the second position at the H-bar assembly to the first position.
According to a third aspect of the invention, there is provided a method for displacing an article during screening using first and second paste screening stations, first and second pallets for holding and transporting an article from a first position at the paste screening station to a second position and a rotatable H-bar assembly for receiving the first and second pallets at the second position, the method comprising the steps of:
screening for the first time the article on each of the pallets with a paste at the first position;
moving the first and second pallets and articles thereon from the first position at the paste screening station to the second position at the H-bar assembly;
receiving the first and second pallets and the articles thereon by the H-bar assembly;
rotating the H-bar assembly a predetermined amount so as to exchange positions of the first and second pallets and the articles thereon;
moving the first and second pallets and the articles thereon from the second position at the H-bar assembly to the first position at the paste screening station; and
screening at the paste screening station for the second time the article on each of the pallets with a paste wherein the article on each of the first and second pallets has not been moved with respect to the first and second pallets between the first and second screenings.